a) Field of the Invention
The present invention relates to an optical low pass filter which is to be used in the so-called electronic image pickup systems adopting image pickup devices as imaging means.
b) Description of the Prior Art
It is widely practised in these days to observe images on TV monitors by using image pickup devices such as CCD sensors as imaging means. Rapid progresses are now being made, for example, in TV cameras and electronic still cameras for observations on the TV monitors. In particular, remarkable development has been made in a field of endoscopes which are to be inserted into human body cavities for medical diagnoses and treatments or into pipes for inspecting corrosion, etc. in inside walls.
Used for practical observations are, for example, electronic endoscopes which have solid-stage image pickup devices at leading ends thereof as shown in FIG. 25 and TV cameras which are equipped with external endoscopes for permitting observations of images formed by the endoscopes on TV monitors as shown in FIG. 26 by reimaging images of objects formed by fiber scopes or non-flexible endoscopes onto solid-state image pickup devices.
The electronic endoscope shown in FIG. 25 is equipped with a manual controller 2, a slender and flexible section 3 to be inserted, and a signal cable 4 which is branched in its course into two cables to be connected to a light source unit 5 and a video signal processing unit 6 respectively. Disposed on a leading end of the section 3 is an observation optical system which comprises an objective lens system 7, a quartz low pass filter 8 and a CCD image sensor 9 as well as an illumination optical system (not shown).
An illumination light bundle emitted from the light source unit 5 is transmitted to the leading end of the endoscope through a light guide fiber bundle (not shown) disposed in the signal cable and endoscope for illuminating an object M with the light bundle.
Rays reflected from the object M are formed, by the objective lens system 7, into an image on the CCD image sensor 9 and converted into electrical signals by the CCD sensor, led through the manual controller 2 and the signal cable 4 to the video signal processing circuit 6 for conversion into video signals and displayed as an image of the object M on a TV monitor for observation.
Further, as shown in FIG. 26 a TV camera which is equipped with a fiber scope and a non-flexible endoscope has an objective lens system 13 disposed on the leading end of a fiber scope 11 so that an image of the object M is formed by the objective lens system 13 on an end surface of incidence of an image guide fiber bundle 12; and is configured so as to transmit the image of the object M to an end surface of emergence of the image guide fiber bundle 12 for allowing observation of the image through an eyepiece 15. In addition, an objective lens system 17 is disposed on a leading end of a non-flexible endoscope 16 and a relay lens system 18 is disposed after the objective lens system 17 so that the image of the object M is transmitted through the relay lens system 18 for observation of the image through an eyepiece 19.
In FIG. 26, the reference numeral 20 represents an adaptor which is to be used for attaching a TV camera head 22 to the eyepiece 14 of the fiber scope or the non-flexible endoscope and has a built-in imaging lens system 21. Disposed in the TV camera head 22 are an optical low pass filter 23 which consists of a plurality of quartz plates and a CCD image sensor 24. The camera head 22 has an output cable connected to a camera control unit 25 comprising a video signal processing circuit. When the adaptor 20 is attached to the eyepiece 14 of the fiber scope or the non-flexible endoscope as described above, rays emerging from an eyepiece lens of the eyepiece 14 are condensed by the imaging lens system 21 and transmit through the optical low pass filter 23 for forming an image of the object M on the CCD image sensor 24. This image is converted into electrical signals, fed to the camera control unit 25 and further converted into video signals for displaying the image of the object M on the TV monitor 26 for observation.
The electronic imaging system described above allows phenomena which are called beat and moire to be produced due to interference between spatial frequency components contained in the image of the object and a sampling period of picture elements disposed on the CCD image sensor since these picture elements have a constant period (sampling period).
As a conventional example which is configured so as to prevent the phenomena of beat and moire, there is known an optical low pass filter which is configured so as to prevent the moire by cutting off the spatial frequency components producing the moire out of spatial frequency components contained in an image of an object by utilizing a spatial frequency limiting filter consisting of a birefringent plate as disclosed by Japanese Utility Model Publication No. Sho 47-18,688.
When a birefringent plate is used as an optical low pass filter as in the case described above, a spatial frequency bandwidth can be limited for a reason which is to be described below. FIG. 27 illustrates a composition of an optical system which is configured so as to form an image of an object by using a combination of an ideal lens component L and a birefringent plate 30. In this case, a light intensity distribution on a spot image formed by the optical system is expressed by the delta functions for the ordinary ray and the extraordinary ray as shown in FIG. 27. When a spacing between these two delta function is represented by t on a coordinates system shown in FIG. 28, Fourier transform of the delta function gives spatial frequency response which is to be obtained by using the birefringent plate. Since the birefringent plate has a modulation frequency function (MTF) which is expressed as MTF=.vertline.cos (.pi.tu).vertline. as shown in FIG. 29, it will be understood that a spatial frequency bandwidth can be limited. Further, the above-mentioned formula means that the MTF of the birefringent plate shown in FIG. 29 has a cutoff frequency 1/2t which can be controlled by controlling the spacing t between the delta functions (spot images). The spacing t can be controlled by adequately setting an angle formed by crystallographic axis of the birefringent plate intersecting with an optical axis of the birefringent plate and thickness of the birefringent plate. In other words, it is possible to prevent the production of the moire by setting the cutoff frequency 1/(2t) shown in FIG. 29 at a spatial frequency of the image of the object at which the moire is to be produced.
In case of the single-plate type color TV cameras which form a main current in practical use in these days, mosaic filter arrays for color encoding are disposed on surfaces of incidence of CCD image sensors. An image pickup device of this type allows not only the arrays of the picture elements but also the arrays of the filter elements to produce the beat, whereby the image pickup device is apt to allow the beat to be remarkable and cannot prevent the production of the moire simply by limiting a spatial frequency bandwidth to such a degree as shown in FIG. 29. Further, in case of the TV camera which is equipped with the endoscope and configured so as to permit observation of the image formed by the endoscope while reimaging an image formed by the fiber scope onto the solid-state image pickup device, an object to be observed through the TV camera is an end surface of an optical image guide fiber as shown in FIGS. 30A and 30B and an image of the object has remarkably intense high-frequency components, whereby the TV camera cannot prevent the production of the moire. Speaking concretely, the optical image guide fiber is composed of optical fibers each of which consists of a core covered with a cladding and an image formed on an end surface of emergence of the image guide fiber consists of a large number of light spots having a repeating period having a pitch PF of the cores. Accordingly, the image formed on the end surface of emergence contains high frequency components which were not contained in an original image formed by the objective lens system but correspond to the repeating period of the above-mentioned light spots. These high-frequency components make it difficult to eliminate the moire.
As a conventional example which is configured so as to correct this defect, there are known optical low pass filters disclosed by Japanese Patents Kokoku Publication No. Hei 4-15,669 and Kokai Publication No. Sho 63-291,026. The optical low pass filters disclosed by these Japanese patents can exhibit an effect to limit spatial frequency bandwidths but have a common defect that these optical low pass filters require high manufacturing costs due to a fact that these filters use expensive birefringent plates.